Recently, with an increase in the information amount that information devices and audiovisual devices are required to process, optical disks that are superior in allowing for easy data access, large capacity data storage, and reduction of device sizes have attracted more attention as recording media, and the recording of information at a higher density has been attempted. For instance, as an optical disk with a higher density, an optical disk having a capacity of approximately 25 GB has been proposed to which the recording and reproduction is performed with the use of a recording/reproduction head whose recording/reproduction laser beam source emits light with a wavelength of approximately 400 nm and whose objective lens for converging a laser beam has a numerical aperture (NA) of 0.85.
The following will describe a configuration of a conventional optical disk and a method for producing the same, while referring to FIGS. 2 to 5. FIGS. 2A to 2F illustrate a method for producing a nickel (Ni) stamper as a substrate mold for use in the production of a conventional optical disk. In the production of the Ni stamper, first of all, a photosensitive film 202 is formed on a glass plate 201 by applying a photosensitive material such as a photoresist thereon (see FIG. 2A), and recording track portions are exposed by optical recording with use of a laser beam 203 (see FIG. 2B). In FIG. 2B, 202a denotes an exposed portion. The photosensitive material in the recording track portions thus exposed is removed through a developing process, and an optical recording master 205 in which a recording track pattern 204 is formed is produced (see FIG. 2C). The pattern of the recording track pattern 204 formed on the photosensitive film 202 is transferred to a conductive film 206 (material: Ni) formed by sputtering or vapor deposition (see FIG. 2D). Furthermore, to increase the rigidity and thickness of the conductive film 206, a Ni plating film 207 is formed (see FIG. 2E). Then, the conductive film 206 and the Ni plating film 207 are detached at an interface between the photosensitive film 202 and the conductive film 206, so that a Ni stamper 208 is produced (see FIG. 2F).
FIG. 3 illustrates a cross section of a thick-substrate transfer-type optical disk as a conventional optical disk. The thick-substrate transfer-type optical disk includes a first substrate 302 having a surface on one side on which recesses/projections are provided as signal pits or recording tracks, a thin film layer 301 provided on the surface of the first substrate 302 on which the recesses/projections are provided, a second substrate 303 arranged facing the first substrate 302, and a transparent layer 304 interposed between the first substrate 302 and the second substrate 303 so as to cause them to adhere with each other.
Signal pits or recording tracks are transferred in a form of recesses onto one side of the first substrate 302 by injection compression molding or the like using the Ni stamper 208 shown in FIG. 2F. The first substrate 302 has a thickness of approximately 1.1 mm. The thin film layer 301 includes a recording film and/or a reflection film, and is formed by sputtering, vapor deposition, or the like on the surface of the first substrate 302 on which signal pits or recording tracks are formed. The second substrate 303 is made of a material that is transparent (has transparency) with respect to recording/reproduction light, and has a thickness of approximately 0.1 mm. The transparent layer 304 is provided to cause the two substrates 302 and 303 to adhere with each other, and is made of an adhesive such as ultraviolet-curable resin or the like.
The recording/reproduction of such a conventional thick-substrate transfer-type optical disk is carried out by projecting a recording/reproduction laser beam thereto from the second substrate 303 side.
Furthermore, FIG. 4 illustrates a cross section of a thin-substrate transfer-type optical disk as another conventional optical disk. The thin-substrate transfer-type optical disk includes a first substrate 402, a signal transfer layer 405 provided on the first substrate 402 that has a surface on one side on which recesses/projections are provided as signal pits or recording tracks, a thin film layer 401 provided on the surface of the signal transfer layer 405 on which the recesses/projections are provided, a second substrate 403 arranged facing the first substrate 402, and a transparent layer 404 interposed between the first substrate 402 and the second substrate 403 so as to cause them to adhere with each other.
The first substrate 402 is made of a material that is transparent (has transparency) with respect to recording/reproduction light, and has a thickness of approximately 0.1 mm. The signal transfer layer 405 is a layer made of an ultraviolet-curable resin, on one of whose surfaces recesses are formed by compression transfer using the Ni stamper 208 shown in FIG. 2F so as to provide signal pits or recording tracks. The compression transfer with the ultraviolet-curable resin is performed by dripping the ultraviolet-curable resin concentrically on the first substrate 402, applying the Ni stamper 208 thereon so that an information surface (surface where the recording pattern 204 is provided) of the Ni stamper 208 faces the first substrate 402, and applying a pressure on the transfer stamper 208. Thus, the spreading of the ultraviolet-curable resin and the transfer of the pattern of the information surface are performed. The thin film layer 401 includes a recording film and/or a reflection film, which are formed by sputtering or vapor deposition on the surface of the signal transfer layer 405 on which signal pits or recording tracks are formed. The second substrate 403 has a thickness of approximately 1.1 mm. The transparent layer 404 is provided to cause the two substrates 402 and 403 to adhere with each other, and is made of an adhesive such as ultraviolet-curable resin or the like.
The recording/reproduction of such a conventional thin-substrate transfer-type optical disk is carried out by projecting a recording/reproduction laser beam thereto from the first substrate 402 side.
The following will describe a configuration of an optical disk in which a phase-change recording material is used for forming the thin film layer so as to form a recording film, so that the optical disk is modified to be a phase-change optical disk. FIG. 5 is an enlarged cross-sectional view illustrating a configuration of the thick-substrate transfer-type optical disk shown in FIG. 3 in which a phase-change recording material is used for forming the thin film layer 301 so as to form a recording film. In the thick-substrate transfer-type optical disk, recording tracks are formed on an information surface 302a of the first substrate 302 as the thick substrate. The recording tracks are grooves formed with recesses/projections at a track pitch 505 of approximately 0.3 μm. Furthermore, a reflection film 501 made of AgPdCu or the like, a dielectric film 502 made of a dielectric material such as ZnS—SiO2, a recording film 503, and a dielectric film 504 made of a dielectric material such as ZnS—SiO2 are formed by sputtering or the like. The recording film 503 is formed by sputtering or the like, with a material such as Ge (germanium), Sb (antimony), and Te (tellurium). The dielectric films 502 and 504 are provided so as to protect the recording film 503 from damage caused by heat, moisture, etc. These reflection film 501, the dielectric films 502 and 504, and the recording film 503 compose the thin film layer 301.
A phase-change recording material makes a transition into an amorphous state in the case where it is cooled abruptly after it is molten, whereas it makes a transition into a crystalline state in the case where it is cooled gradually after it is heated. An optical disk having a recording film that is made of such a phase-change recording material by taking advantage of the foregoing property makes reversible transitions between the crystalline state and the amorphous state, thereby being capable of overwriting. The reproduction of signals on the optical disk is performed in the following manner: while focusing control and tracking control are carried out so that a reproduction laser beam having a small constant intensity is positioned on a groove track in which signals are recorded, a change in an amount of light reflected from the optical disk is detected by a photodetector device, utilizing a property such that amorphous portions as recording marks and crystalline portions other than the recording marks have different reactances or different transmittances. It should be noted that FIG. 5 illustrates an example of the thick-substrate transfer-type optical disk shown in FIG. 3 modified by using a phase-change recording material to form the thin film layer 301, and in the case of the thin-substrate transfer-type optical disk shown in FIG. 4, it also is possible to form the thin film layer 401 using a phase-change recording material.
A phase-change optical disk in which the thin film layer 301 or 401 is formed with a phase-change recording material is described as the foregoing conventional optical disk, but it is possible to provide an optical disk that is capable of reproduction solely by forming signal pits on the substrate and forming a reflection film in the thin film layer 301 or 401.
However, with an increase in the density of an optical recording medium, the reproduction of an optical recording medium is affected by fine recesses/projections on the optical recording medium as noise sources more than before. In the case where, like a Ni stamper used in the production of a conventional optical disk, a Ni stamper is produced through the laser exposure of a photosensitive material such as a photoresist for recording signal pits or recording tracks, which is followed by the development, the sputtering and the plating, the roughness of the photosensitive material and the like due to the coarseness of the photosensitive material or caused by a developer is transferred to the Ni stamper from the photosensitive material surface, thereby leaving fine recesses/projections on the Ni stamper. Therefore, in the case where an optical disk is produced using such a Ni stamper, the fine recesses/projections become noise sources. In the case where the injection compression molding or the compression transfer of an ultraviolet-curable resin is used for transferring a surface state of a transfer stamper to a substrate, the surface roughness of the stamper is transferred to the substrate in detail by compression. Therefore, an optical disk including such a substrate has high reproduction noise. Furthermore, in the foregoing production by the injection compression molding or compression transfer with use of an ultraviolet-curable resin in which the foregoing Ni stamper is employed, it is difficult to form, on a substrate surface, signal pits or recording tracks with recesses/projections that have a uniform depth/height (level difference between bottoms of the recesses and tops of the projections) thereby providing high reproducibility, or alternatively, to make uniform within a disk surface a sum of thicknesses of a thin substrate and an ultraviolet-curable resin that determines the conversion of a recording/reproduction laser light upon the recording/reproduction of a signal surface of a thin-substrate transfer-type optical disk.